In general, electric resistance welded pipes (hereinafter, referred to as electric resistance welded steel pipes) are manufactured by transporting a metal strip (including a metal sheet) such as a steel strip in the lengthwise direction while shaping it into a pipe, and continuously butt welding both end parts of the metal strip in the width direction along the lengthwise direction using a method such as high-frequency induction heat-pressure welding and resistance heat-pressure welding. In a process of manufacturing the electric resistance welded steel pipe, it is important to prevent foreign matters such as oxides from remaining in a butt-welded part in view of the quality control of ensuring of the strength of the butt-welded part, and the like. For the quality control of the electric resistance welded steel pipe, a technique in which flaws in the electric resistance welded steel pipe are detected while attention is mainly being focused on the butt-welded part using an ultrasonic inspection device, as disclosed in Non-Patent Literature 1, has spread.
For quality control in manufacturing of the electric resistance welded steel pipe, a technique has been proposed, in which the shape of a welded bead part is measured using an optical cutting method and the feature amount of the welded bead part is calculated from a measurement result to be used for the quality control, as disclosed in Non-Patent Literature 1. With this technique, the feature amount having a correlation with metal flow in a cross section is calculated online, thereby omitting a process of observing the cross section by interrupting a welding process to sample the butt-welded part. Furthermore, as another technique related to the quality control, there is a technique in which the feature amount of a planar shape is calculated from an image of a butt-welded part that has been shot by an imaging device and whether the feature amount of the planar shape is within a control range is determined, as disclosed in Patent Literature 2.